Optical scanning measurement of dimensions with a double-concave mirror

ABSTRACT

An apparatus (5) for measuring the dimensions of an object (1) where a measuring carriage (6) moves the object at a distance thereabove. A polygonal mirror unit (32) receives laser light (16) and reflects it in a fan-shaped form toward a stationary angled mirror unit (37, 38) which directs the light towards a double-curved stationary mirror unit (39) which reflects the beam onto the object. Mirrors (40) of the polygonal unit are adjustable.

FIELD OF THE INVENTION

The present invention relates to an apparatus for measuring the distanceto one or more areas of an object in order to determine one or moredimensions of said object, comprising means for directing a light beamtowards the object, means for registering the light beams that arereflected from the object, means for calculating the time it takes forthe beams to travel to and from the object, and means causing the lightbeam to sweep over the object perpendicular in relation to a fixedreference plane during the entire sweeping operation, and with relativemovement between the apparatus and the object to be measured, the lightthat is reflected by the object being transmitted back to the lightregistration means via said light beam sweeping means, and said lightbeam sweeping means including a stationary mirror which is arced in itslongitudinal direction, and also means, e.g. a rotatable mirror, whichcauses the light beam to move over the stationary mirror.

PROBLEMS OF THE RELATED TECHNOLOGY AND OBJECTS OF THE INVENTION

An apparatus of this kind is known from U.S. Pat. 4,996,440. This knownsolution includes a pitch in the scanning arc which corresponds to theradius of curvature of the stationary mirror. This means that themeasuring apparatus must move over an unduly long distance whichcorresponds to the length of the object plus twice the length of saidradius of curvature.

It is therefore an objective of the present invention to reduce theradius of curvature and thereby make the measuring apparatus as compactas possible.

In the known solutions, it is intended that the distance be measuredfrom the measuring apparatus and down towards the underlying supportalong parallel, vertical light paths. However, in is not always easy toobtain such vertical and parallel paths of measurement. This makes greatdemands on the measuring optics. With the present invention, it is thusintended to obtain parallel paths of measurement of this kind, and alsoto ensure that it is possible to continuously calibrate the distancemeasuring apparatus while in operation.

One objective of measuring by means of parallel, vertical lines is,inter alia, that the position of the point of measurement will not bedependent upon the vertical distance of movement of the light. Inaddition, it is desirable that a reflecting component in the surface ofthe object will at one point at least, namely the highest point, reflectthe light back to the measuring apparatus. Furthermore, a principalobjective of the present invention is to avoid fields of shadow.

One of the known problems with distance reading apparatus is to be ableto shield a light detector in such a way that one prevents orcompensates for unwanted reflected rays or so-called scattered light insuch a way that light of this kind does not reach the detector. Thepresent invention provides a solution which enables this problem to beovercome.

Moreover, to be able to finely adjust the position and/or focus of alens unit of a diode laser in a simple way is known to be a problem. Thepresent invention aims also to provide a means to enable this to beaccomplished.

BRIEF DESCRIPTION OF THE DRAWING

The characteristic features of the invention will become apparent in thepatent claims hereinbelow and also in the specification with referenceto the enclosed drawings.

FIG. 1 illustrates one use of the invention in a typical workingsituation.

FIG. 2 illustrates the problems linked with "fields of shadow".

FIG. 3 illustrates the measuring principle of a laser distance reader.

FIGS. 4A and 4B show the pitch and gauge length in connection with theprior art, e.g., as disclosed in U.S. Pat. 4,996,440, and FIGS. 5A and5B show the pitch of the scanning arc and the gauge length, according tothe invention.

FIGS. 6, 7, 8, 9 and 10 show in perspective view, in side view, planview, side view and from above, respectively, details of the measuringprinciple according to the invention.

FIGS. 11 and 12 illustrate in detail a polygonal mirror unit seen frombelow and in partial section, respectively.

FIG. 13 shows the deflection of laser light by means of the mirror unit.

FIG. 14 illustrates the problems connected with stray light.

FIG. 15 discloses geometrical considerations connected to the problemsof stray light.

FIGS. 16 and 17 show a first embodiment of a stray light detector.

FIG. 18 shows a modification of the stray light detector in FIGS. 16 and17.

FIG. 19 shows a block diagram of a circuit for cancelling stray light.

FIG. 20 is a front view of a first embodiment of an apparatus for finelyadjusting position and/or focus of the lens unit of a diode laser.

FIG. 21 shows the section XXI--XXI in FIG. 20.

FIG. 22 shows in front view a modification of the apparatus in FIGS. 20and FIG. 21.

FIG. 23 shows the section XXIII--XXIII in FIG. 22.

FIG. 24 shows the measuring apparatus according to the invention placedin a carriage that can be moved back and forth.

FIG. 25 show a detail of the carriage in FIG. 24 for the moving thereofand asspcoated cabling.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The physical object 1 which is to be measured is placed by means of anoperator 2 on a table 3, a weighing scale or another stable, invariablesurface. The measuring apparatus, in FIG. 1 indicated in general bymeans of the reference numeral 5, is advised that a measurement isrequired by means of a start signal given via a push button, foot pedal4, or a command across a serial interface from an external computer (notshown). The measuring apparatus 5 is in a measuring carriage 6, and saidmeasuring carriage 6 is made to move over the object 1 along a guidingprofile 7, whereby the dimensions of the object are read. A short timeafter the carriage 6 has passed over the object 1, the measureddimensions and/or parts thereof, e.g., the volume of the object, will beshown on a display 8. The result of the measurement may optionally alsobe fed to said computer. A print-out 9 may optionally also be providedvia a printer 10. This print-out can, for instance, be in alphanumericform or in the form of bar code, or optionally in the form of both. Theprint-out may optionally be of a kind such that it can be adhered to theobject, so that the dimensions of the object are thus unambiguouslydefined in connection with possible later handling of the object.

After an output and/or print-out has been given the measuring apparatuswill be ready to measure a new object.

FIG. 2 is related to problems of so-called "fields of shadow". In theapplication of volume measuring for allocating tariffs for freightservices, the regulations are drawn up in such a way that payment has tobe made for the smallest right-angled rectangular box or crate in whichthe object can be placed. If the path of measurement or the path of thelight beam deviates from 90° towards the underlying support 3, a fieldof shadow 11 will occur, as illustrated in FIG. 2. The measuringapparatus will be unable to detect the content of said field of shadow.The angle that the scanning light beam 12 forms with the vertical, theangle α, should thus be as close to 0° as possible. The maximum error ofmeasurement will depend upon how much there may be within a shadow areaof this kind and that cannot be detected. As an example, let us assumethat one has a package that is of a height equal to 700 mm and aspecified accuracy of measurement of the measuring apparatus that isequal to 5 mm. With a solution of this kind the angle α must deviatewith <5/700 rad, equal to approx. 7.6 milliradians. If one wishes totake into consideration deviations which may go each in their owndirection on either side of the package, the angle must be <3.5milliradians.

FIG. 3 shows the measuring principle for a laser distance reader,generally denoted by the reference numeral 13. An oscillator 14modulates a laser diode 15 with an FM frequency (e.g. 82 MHz). The laserlight which is emitted from the laser diode 15 is denoted by referencenumeral 16 and passes via a mirror unit 17 downwards at right anglestowards the underlying support 3 on which an object may be placed. Lightreflected back is marked with the reference numeral 18 and travels pastthe mirror unit 17 via optics 19 to an optical sensor 20, e.g. aphotodiode which is capable of registering the received laser light.This received light signal is transferred via a connection 21 to anamplifier 22 which has automatic gain control (AGC). A signal isproduced at the first outlet 23 from said amplifier which ischaracteristic of the received luminous intensity. A second outlet 24from the amplifier 22 is connected to a first input 25 on a phasemeasuring device 26, and an outlet 27 on the oscillator 14 is connectedto a second input 28 on the phase measuring device 26. The distancemeasuring apparatus will measure the distance to the object 1 bymeasuring the time T1 that the light takes from when the laser beamleaves the distance measuring apparatus, hits the object 1, and isreceived as reflected light by the sensor 20. This time is measured bymeasuring, in the phase measuring device 26, the phase differencebetween the emitted light, as registered at the input 28, and the lightreflected back, as registered at the input 25 of the phase measuringdevice 26. Characteristic distance values are then produced at theoutput of the phase measuring device 29.

U.S. Pat. 4,996,440 makes known the use of a cone segment mirror surfaceto deflect the scanning beam to scan at right angles downwards to theunderlying support. With a solution of this kind, as indicated in FIGS.4A and 4B, the necessary gauge length for the measuring carriage willhave the value L1 which is equal to twice the radius R1 of the conicalmirror 30, as shown in FIG. 4B and also the length L2 of the object asshown in FIG. A.

In the present invention, as is made apparent in FIGS. 5A and 5B,instead of the rotating plane mirror 31 in FIG. 4B, a polygonal mirrorunit 32 is used. The mirror unit 32 is, in the preferred embodiment,made of six mirror surfaces arranged on the six sides of the polygon.According to the present invention, instead of the conical mirror 30, amodified paraboloid is used, i.e., a double curved mirror or a parabola,the arc of which having a radius of curvature which is modified somewhatover the length of the mirror and being greatest at the ends of themirror. In FIG. 5B, the modified paraboloid mirror is marked with thereference numeral 33. Here R₃ denotes the length of the measuringcarriage which is increased relative to the known R₁ value. R₂ is thedistance between the center of the mirror surfaces of the polygonalmirror unit 32 and the mid-point of the double curved mirror 33, as isseen in the drawing. R₄ denotes the radius of curvature of the mirror33. However, it should be pointed out that R₂ will change gradually asthe mirror 32 rotates and that this is compensated by the design of thedouble curved mirror 33.

It is sideways component d₁, in particular, that is the problem whenusing the polygonal mirror, see FIG. 13. The shifting d₂ of the footpoint along the mirror surface does not, however, cause such greatproblems. To compensate for said shifting d₁ in particular, R₄ will begreater at the ends of the mirror 33 than at the midpoint 33'. Thus, atsaid ends in particular R₄ >R₂, see FIG. 5B. Furthermore, the pitch 35of the scanning path 34 will be considerably smaller, preferably approx.15-30% of the radius of curvature or the pitch 36 which is made possibleaccording to the solution shown in FIG. 4A, but on the other hand thelength of measuring carriage increases.

However, this means that the effective measuring length that themeasuring carriage must move will be equal to L3, which is twice thepitch 35 plus the length L2 of the object 1. The measuring length willbe related to effective scanning time over the object.

As will be seen from FIG. 4A and 4B, the conical mirror 30 spans over anangle which is almost equal to 180°. One problem with a solution of thiskind is that said pitch 36 of this arc will be large. With the solutionaccording to the invention, one achieves a reduction in the length ofthe apparatus or guiding rail 7 when the pitch 35 is reduced. Thescanning angle for the fan-shaped scan will thus also be smaller. Thisin turn means that it is possible to use a plurality of mirror surfacesin the polygon and rotate said polygon at a lower speed of rotation forthe same scanning frequency as is used in the known solution accordingto FIG. 4. The most substantial disadvantage with the solution which isoutlined in FIG. 5 is that the length of the measuring carriage in thedirection of movement will be greater owing to the increased radiusbetween the polygonal mirror unit and the double curved mirror 33.

As will be described in more detail in connection with FIGS. 6 to 10,the present invention aims to make the dimensions of the measuringcarriage as small as possible.

Before FIGS. 6 to 8 are described in more detail, a brief reference willbe made to FIG. 9. In the solution illustrated therein, the distancebetween the centre of the mirror surfaces of the polygonal mirror unitand the mid-point of the double curved mirror 33 is equal to R₂ as shownin FIG. 5B. The length of the measuring carriage will, however, beconsiderably reduced in that the scanning beam, which spreads out in afan shape, will be folded 90° twice by means of two mirrors 37 and 38which form an angle of 90° in relation to one another, said light beambeing reflected from the mirror unit 32 towards the mirror 37, thence tothe mirror 38, and on further to a double curved mirror 39 so that, inthe example shown in FIG. 7, the distance between a centre of the mirrorsurface of the polygonal mirror unit and the mid-point of the doublecurved mirror 39 has a distance R₅. In this way one achieves aconsiderable reduction of the total dimensions of the apparatus. Inaddition to giving rise to a more compact measuring carriage, which issomewhat at the expense of the height of the carriage, this solutiongroups the optics in two sets of mirror surfaces which are mounteddirectly above/below one another. As long as an even number of mirrorsurfaces are closely connected to one another at fixed angles inrelation to one another, a rotation or translation of the two connectedmirrors, which together form one system, will be of no consequence forthe angle at which the light leaves the mirrors taken as a whole. Thus,the two folding mirrors 37 and 38 are connected to one another in onesystem and the polygonal mirror unit 32 and the double curved mirror 39are connected together in a second system within the present apparatus.

The laser distance measuring device 13 will, by means of its laser, emita light beam 16 in the direction of the mirror 17, whence the light beam16 is reflected in the direction of the rotating polygonal mirror unit32. This unit has, for instance, six mirror surfaces 40 mounted on theside surfaces of the polygon. Each mirror has been given a size that hasbeen determined by the aperture of the receiving optical means 19. Thelaser light will be dispersed in a fan shape from the polygonal mirrorunit 32 as this rotates. This fan-shaped light will be folded in saidmirrors 37 and 38 which are mounted at 90° relative to one another. Thescanning light beam or laser light fan will thus hit a large doublecurved deflecting mirror 39. This mirror will deflect the received lightbeam 90° so that the light scans at right angles downwards to theunderlying support 3 which is found under the measuring apparatus. Thislight that has been reflected from the object will be, as explainedearlier, detected and the phase difference between the emitted laserlight and the received laser light will, as previously mentioned, formthe basis for calculating successive distances to the object 1 which ison the support 3.

As will be understood immediately, the light which is reflected from theobject will be led via the double curved mirror 39 to the lower foldingmirror 38, thence to the upper folding mirror 37, thence to thepolygonal mirror unit 32, and thence via the optical means 19 to thedetector 20.

As will be understood from FIG. 6, the polygonal mirror unit 32 will bedriven by a motor 41, where the mirror unit constitutes a part of themotor rotor, and where the motor central stator is via a suspension orshaft 42 rigidly connected to a framework or other mounting means in themeasuring carriage.

A laser distance measuring apparatus, such as the one described andillustrated here, will be capable of being exposed to a certain drift inthe measurement values over time, for example because of temperaturechanges, aging, variation between models, etc. It will therefore be bothdesirable and necessary to calibrate the distance measuring apparatuswhilst it is in operation. For this purpose it is proposed, according tothe invention, to provide a mirror 43 at the end of the double curvedmirror 39, i.e. between a trigger diode and the mirror 39. The lightwill be deflected here so as to pass through a window 45 (seen in FIG.10) and hit an end 46 of the guide rail or guide profile 7. A smallplate 47 which has been given a grey toned pattern is positioned at theend 46 and from here measurements are taken during each scan. When thecarriage 6 moves along the rail 7 the position of the carriage ismeasured by means of a pulse generator on the drive motor of thecarriage. This pulse generator operates a counter circuit which will bezero set at the end position of the rail. Thus, from this counterabsolute information is obtained with regard to the position of thecarriage and this is used for calibrating the distance measuringapparatus. The grey toned pattern on the plate will provide measurementsof different luminous intensity.

Said trigger diode 44 and also 48, as shown in FIGS. 8 and 10, will emita signal when the light beam meets said diodes, where starting andstopping of a scan will be indicated. This will also give a fairlyreliable determination of the momentary position of the light beam alongthe double curved mirror 39 when the speed of the motor 41 is known oris constant, so that it can be measured exactly by means of the triggerdiodes.

In the present invention, a polygonal mirror unit 32 is used, the taskof which being to spread out the laser beam and to pick up the lightspread towards the receiver in a fan shape, as previously described, sothat the object to be measured is scanned in one axial direction.However, it is considerably difficult to construct a polygonal mirrorunit of this kind with precision sufficient to ensure that the path oflight which is generated by each of the six mirrors (in the chosenexemplary embodiment) are as identical as possible. As a minimumrequirement, the paths of light must be so precise that the scanningpaths will be monotonously ascending with the direction of movement ofthe measuring carriage.

The diameter of the polygonal unit is determined by a necessary aperturefor the receiving lenses and the number of mirrors that are necessaryfor the desired scanning angle. The angle of the mirrors along thedirection of rotation is assumed to be non-critical, since this willonly cause a time displacement of the scan. This could be compensatedfor by the light passing said trigger diode 44 immediately prior to thestart of the scan, and said trigger diode 49 immediately after the endof the double curved mirror 39 thereby registering the time of the endof the scan.

The angle of the mirror transverse to the direction of rotation musthowever deviate very little from mirror to mirror, and in the presentembodiment of the invention this angle deviation must typically be <1milliradian.

The angle of the mirrors will be adjustable, according to the invention,by means of an apparatus such as the one in FIGS. 11 and 12. Thepolygonal unit here is made in that the shaft 49 of the driving motor 41and thus the stator of the motor is secured to a holder 50 for thepolygonal mirror unit. The motor housing will thus be connected to themirrors 40 which are a part of the unit 32, said motor housing rotatingtogether with the mirrors 40.

The mirrors are deposited in a holder 51 which fixes the under side ofthe mirrors at two points 52 and 53. The upper side of the mirrors aresupported at one point by a flexible metal plate 54. The plate 54 has,moreover, two punched-out tongues 55 and 56 which project from thecentre of the metal plate 54 and come to bear against the back of themirror in order to press the mirror out towards its abutment pointsuppermost and lowermost. A screw connection 57 causes said mirror bottomholder 51 and said flexible metal plate 54 to be held together. Bytightening the screw connection 57, the metal plate 54 will be benttogether, whereby the upper side of the mirror 40 will be drawn in thedirection of the mid-point of the polygonal unit. If the screw isloosened, the mirror will move outwards as a result of the spring effectof said metal plate 54. The form of the metal plate makes it possible toobtain considerable exchange between the downwards bending of the plateand the position adjustment of the mirror. The polygonal mirror unit isconnected to the laser distance measuring apparatus 13, which can befirmly fixed to the holder 50 in such a way that the polygonal unit andthe laser distance measuring apparatus thereby have their positionrelative to one another fixed. By installing the distance measuringapparatus/polygonal unit in a bench, and turning the laser on, themirrors 40 in the polygonal unit will be adjustable to the correct andpossibly identical angles by manually rotating the polygonal unit 32 andobserving the light path of the laser on a screen at a distance that issufficient to make possible an accurate adjustment of the position ofthe paths of light.

A pin 58 is arranged on the polygonal unit which extends down from thepolygonal unit and will interrupt a light beam when it passes a lightfork. Said pin is used to identify the position of the polygon bycommunication with a computer so that the computer knows which of themirrors is used in the scan at any point of time. Thus the computer willbe able to include possible remaining errors in the mirror angles of thepolygonal unit so that such errors can be included in the calibratingalgorithms of the computer.

An ideal, fan-shaped scan is generated by a mirror which rotates aroundan axis which is placed in the centre of the mirror in the plane of thereflecting coating. However, it is only possible to achieve this byusing one or two mirror surfaces (double-sided mirrors). In the presentinvention, the scan constitutes approx. 60°. If a double-sided mirror isused, only twice 30° of a 360°-rotation could be used which would giverise to poor efficiency in terms of time.

To make better use of the distance measuring apparatus, according to thepreferred yet not limitative embodiment of the invention, six mirrorsurfaces are used which are assembled in a polygon, where the diameterof the polygon is determined by the size of each mirror. The mirrorsurfaces will, in this way, be displaced from the centre of rotation andthis will give rise to a fan-shaped scan where the centre of rotationwill, in reality, move during rotation. The consequence of this in thepresent case will be that the scanning beams which sweep over the objectwill not be parallel if a paraboloid is used as a double curved mirror.In the present invention, the form of this mirror is therefore modifiedin relation to a rotated parabola, so that the light will travelparallel down towards the object 1. This is done in practice by changingthe radius value and also the position of the radius centre for therotated parabola sufficiently along the mirror, the opposite of whatwill happen to the polygon after the polygon scans over the doublecurved mirror, whereby the error will be compensated.

FIG. 13 shows in more detail how the foot of the reflected beam movesalong the mirror surface (and the position of the centre of thefan-shaped scan) as the mirror surface 40 rotates. At a first position59 for the mirror unit 32 the distance between the foot point of thebeam and the centre of the mirror unit is d₃. Furthermore, the anglebetween the incident beam 16 and the reflected beam 18 is equal toapprox. 90°. When the polygonal unit 32 is at position 60 (which can beseen in FIG. 13) the reflection point or foot point of the incident beam16 will have changed so that the distance from said centre will now haveincreased to d₄. The angle between the incident beam 16 and the exitingbeam 18 is in this case β, e.g. 30°. Moving the centre 40' of the mirrorsurface 40 over the distance d₂ thus causes the foot point of thereflected beam to move over the distance d₁ =d₄ -d₃. It is preciselythis last movement that is problematic.

It will thus be understood that the movement (sideways and in/out) ofthe centre of rotation must be compensated, and this happens in that theradius of curvature of the double curved mirror is varied along thelength thereof.

As is made clear in FIG. 1, the measuring carriage 6 has a window 61.This window which comprises a glass plate is necessary in order toprevent a build up of dust inside the measuring apparatus 5, i.e. insidethe carriage 6. A build up of dust could, to a great extent, affect theresult of the measurement in an unfavourable direction and wouldmoreover require constant cleaning. It is obvious that it would besimpler to clean only the surface of the window 61 on the outside of thecarriage 6. As a result of dust and scratches, this window will be thegreatest image for stray light. This window will represent the largestproblem when it comes to shielding.

However, when using a window glass plate of this kind, unwanted lightreflexes may occur therefrom, so-called stray light. The problemsassociated with this will be explained in more detail in connection withFIGS. 14 and 15 and thereafter also in connection with FIGS. 16 to 19.

A laser distance measuring apparatus 13 measures distance by measuringphase difference between emitted and reflected light from the object 1.This phase difference will be influenced by optical devices, if any,such as said glass plate 61, through which the light passes on its wayand which partially reflects light back to the light sensor in the laserdistance measuring apparatus 13. A window surface 61 would typicallyreflect 1-4% of the light and 96-99% of the light would pass through thewindow surface, although dependent upon the material used in the window,coating thereof, if any, etc. If the window is positioned perpendicularto the beam direction, the reflection will be captured by the receiverand will modify the distance measurement in such a way as will beunderstood in more detail by looking at FIG. 15 together with thefollowing mathematical expressions:

    S=k sin(2πft+θ.sub.3)=A sin(2πft+θ.sub.1)+B sin(2πft+θ.sub.2)

    Ax=A sin(θ.sub.1), Ay=A cos(θ.sub.1),

    Bx=B sin(θ.sub.2), By=B cos(θ.sub.2)

    kx=k sin(θ.sub.3), ky=k cos(θ.sub.3)

    k=((kx).sup.2 +(ky).sup.2).sup.1/2

    θ.sub.3 =A tan(kx/ky)=A tan((A sin(θ.sub.1)+B sin(θ.sub.2))/(A cos(θ.sub.1)+B cos(θ.sub.2))

One will see here that amplitude and phase displacement of the object tobe measured is described as B₁ θ₂ and that the stray light from thewindow is denoted A₁ θ₁. The signal which is measured will thereby bemade a vectorial sum of these two signals, viz., k₁ θ₃. If θ₁ and θ₂have a phase difference of 90°, the error will have a maximum effect.

Whilst the emitted light typically would be in the range of 1 to 10 mW,the light reflected back from the object to be measured would typicallybe in the range 50 to 1000 nW. The stray light must typically be <1/100of the retro-reflected light in order not to affect the accuracy of themeasurements. This means that the stray light from the optical surfaceswhich is reflected back to the light sensor must be less than onemillionth of the emitted light energy.

It is therefore essential to be able to detect both stray light andlight which is received from the object being measured in order to beable to cancel the effect of the stray light source. The theory behindthis will be understood in more detail by looking briefly at FIG. 19 andthe explanation hereinbelow.

Light from the source of stray light is denoted L_(s) and light from theobject being measured is designated L_(m). Furthermore, suppose that thelight energy on the main detector is L_(H) and on the stray lightdetector is L_(B). The following is thus obtained:

    L.sub.H =k.sub.1 *L.sub.s +k.sub.2 *L.sub.m

    L.sub.B =k.sub.3 *L.sub.s +k.sub.4 *L.sub.m

wherein k₁ <<k₂ and k₃ >>k₄

Now suppose that the electric response to these signals in the lightsensor 62 and 63 is summed electrically in a summation circuit 64 sothat:

    L.sub.tot =L.sub.H -k×L.sub.B

Prior to being applied to the summation circuit, the stray light signalpasses a phase and amplifying adjustment circuit 65 which has anamplification factor k.

By setting the amplification factor k=k₁ /k₃, L_(tot) =k₁ *L₃ +k₂ *L_(m)-k₁ *L_(S) -k₄ *k₁ /k₃ *L_(m) or since k₁, k₄ <<k₃, it is obtained:

    L.sub.tot =k.sub.2 *L.sub.m

In the electrical arrangement in FIG. 19 the signals are summed in thatphase and amplification of the signal from the stray light detector areadjusted until the signal from a reflector (not shown), which is placedat a distance which corresponds to the outer source of stray light,provides zero signal from the circuit. The distance measuring apparatuswill then be insensitive to stray light which is reflected at thisdistance.

Since the quantity of stray light on the stray light detector 63 isgreater than on the main detector 62, only a small part of the signal inthe stray light detector will be inter-mixed with the main signal. Noisefrom the stray light detector and its amplifier will therefore only to asmall degree affect the signal--noise ratio of the main detector. Themain detector consists of a tuned LC-circuit 66 which serves as a filterand an amplifier 67 which has a high signal--noise ratio. The signal atthe output 68 will thus be representative for that light which isreflected from the object 1 and from the underlying support 3.

In order to be able to detect both main light and stray light, it isessential to be able to separate the two types of light optically.

In FIG. 16 a so-called wedge lens 69 is shown which refracts the lightin the central part of the receiving lens 70 to the side. The lens 71 isan additional lens and has no significance for the understanding of howthe main light and the stray light are separated in the solution shownhere. Shields 72 and 73 are provided in order to ensure that only lightin the central parts of the lens area hits the detectors 62 and 63. Byusing the wedge lens C an optical shielding is achieved, but it is, asindicated above, not required that one shields sufficiently for onehundred percent elimination of stray light from the outermost source ofstray light in the apparatus, namely the window 61 in the measuringcarriage. The light sensor for the main light, also here denoted by thereference numeral 62, may be a PIN diode which is positioned at a focaldistance which corresponds to the area of measurement, and willsubstantially receive light from the object being measured, and becauseof the wedge lens 69 receive a reduced part of stray light from theoutermost source of stay light.

As will be seen in more detail from FIG. 17, the wedge lens 69 willrefract light from the centre of the receiving lens 70, 71 out to theside of the main light detector 62 to the stray light sensor 63 which ispositioned at a focal distance which corresponds to the outermost sourceof stray light, in this case the glass plate 61. This stray light sensorwill, in the main, receive scattered light, and because of the focaldistance, a reduced part of the main light.

In FIG. 18 a modification of the solution in FIGS. 16 and 17 is shownwhere a lens 74 is placed in connection with the shield 75 and where thewedge shaped lens 69 is placed at a distance from the lens 74, incontrast to the solution in FIGS. 16 and 17. Moreover, the mirror 17which deflects the light beam from the light source 15 is alsopositioned at a distance from the wedge lens 69. The manner of operationof this solution which is illustrated in FIG. 18 will, however,correspond to that which has just been described in connection withFIGS. 16 and 17.

In connection with a measuring apparatus of the present type or othermeasuring apparatus where accurate positioning of the emitted laser beamis required, it is essential that the focusing and adjustment of a diodelaser can be carried out in a simple, yet accurate manner.

In this connection closer reference shall be made to FIGS. 20 and 21. Itis expedient to equip a laser diode 76 with a fibre lens 77 forcollimating the laser light. The fibre lens can be of the "Selfoc" kind.The positioning of this lens is important for achieving a good qualityof the laser spot which is reproduced on the object being measured. Bothsideways position, orientation and distance in relation to the diodelaser 76 are important, and typically a positioning must be carried outwith a precision which is better than 50 μm. For this purpose, accordingto the invention, a fine positioning of the fibre lens 77 is proposed inorder to finely adjust the direction of the emitted laser light, whichentails a further requirement of accurate positioning of the lens 77,typically with an accuracy equal to 1 μm.

To be able to adjust focus, the distance between the fibre lens 77 andthe laser diode 76 must be finely adjusted. The fibre lens 76 is put ina V-shaped groove 78 and is held in place by a spring 79. The lens 77 isoutermost on an arm 80 which is resilient about a centre of rotation 81which is so far away that the lens can be said to be displaced parallelto and from the laser. A screw 82 is fixed in the arm 80 and is drawnback and forth by a nut 83 against the tension effect of an O-ring 84.The focus of the laser will thus be adjusted by turning the nut 83.

To obtain side positioning of the lens in relation to the laser diode,the retainer plate 80 for the lens 77 will be solidly secured in aretainer plate 85 by means of two screws 86 and 87. The retainer plate85 is provided with two slots 88 and 89 which form arms 90, 91 and 92.The arms 90 and 91 are interconnected at point 93 and the arms 91 and 92are connected at point 94. The lens 77 is attached to the arm 92 via theretainer plate 80 and the screw connection 86, 87.

By adjusting a screw 95, the arm 92 will rotate elastically around thepoint of rotation 94. By adjusting a screw 96, the arm 90, and also thearm 92, will rotate elastically around the point 93. These adjustmentscrews 95 and 96 will thus displace the lens 77 sideways relative to thelaser 76 along two paths of rotation which are at 90° to one another.One thus achieves independent sideways adjustment along the two axialdirections. Owing to the difference in distance from the points ofrotation to the lens and to the screws, a considerable exchange factoris achieved which makes possible the fine positioning of the lens.

In order for the emitted laser light to coincide exactly with theoptical axis of the receiving optical means, the side position of thefibre lens will be finely adjusted until maximum light signal from anobject to be measured is received on the receiver diode 62. By using ashield 73, as is shown in FIG. 16, stray light will be prevented frombeing scattered inside the distance measurement housing and from beingable to reach the receiving diode. Possibly, the shield 73 can bereplaced by a tube (not shown) the axis of which coincides with the lens77.

A variant of the adjustment apparatus which is shown in FIGS. 20 and 21can be seen in the attached FIGS. 22 and 23.

To adjust the focus of the laser diode 97, the distance between thefibre lens 98 and the laser diode must be finely adjusted.

The fibre lens 98 is put in an almost V-shaped groove 99 and is securedwith a UV-curable glue or similar adhesive. The lens 98 is in aretaining plate 100 which is designed to have two slots 101 and 102which form arms 103, 104 and 105. Furthermore, in retainer plate 100 anadditional slot 106 is arranged causing the formation of two additionalarms 107 and 108. The arms 103 and 107 are rigidly connected to oneanother and the arms 105 and 108 are also rigidly connected to oneanother. The arms 104 and 105 are connected to one another at a point ofrotation 109 and the set of arms 105, 108 are rotatably connected to theset of arms 100, 107 via a rotational connection 110. As can be seenfrom FIG. 22, the lens 99 is fixed to the arm 104 close to the point ofrotation 109.

By adjusting a screw 111, the arm 104 will rotate elastically about thepoint of rotation 109. By adjusting a screw 112, the arm 105, andsimilarly the arm 104, will rotate elastically about the point ofrotation 110. These adjustment screws will displace the lens sidewaysrelative to the laser 97 along two paths of rotation which are at 90° toone another. One thus achieves independent sideways adjustment along twoaxial directions. Because of the difference in distance from the pointsof rotation to the lens and to the screws, a considerable exchangefactor is achieved which enables the lens to be finely positioned. Thelaser diode 97 is mounted by means of screw connections having retainerdiscs 113 and 114 in an arm 115 which via a stay 116 is secured to aframe 117. The stay or spacer 116 can optionally be unitary with the arm115 and the frame component 117, eg, made as an extruded profile, or thearm 115, the stay 116 and the frame component 117 can be made asseparate parts which are fastened together by means of a screwconnection or by gluing. Adjustment of the focus between the laser diode97 and the fibre lens 98, i.e. the relative distance between the twoelements, will be carried out by means of a screw 118 which extendsbetween the frame component 117 and the arm 115. Bending of the arm 115will take place near the stay or spacer 116, and because of the shortdistance of movement of the laser diode 97 relative to the fibre lens98, seen in relation to the length of the arm 115, the movement betweenthe two elements 97 and 98 will be almost parallel.

The arm 115 is attached, via the screw connections 113 and 114, to acircuit board 119 which is used, for instance, for the electronicsinvolved in controlling the laser diode 97. However, it will beunderstood that the circuit board 119 has no significance for theappreciation of the adjustment possibilities for the fibre lens 98 andthe laser diode 97.

The measuring apparatus, which is described, inter alia, in connectionwith FIGS. 6 to 9, can also be seen in FIG. 24. The measuring apparatus5 with its carriage 6 is mounted on an aluminium profile 120 which hastwo grooves where, by means of cold welding, two steel rails are placed,one rail 121 on the upper side and a second rail 122 on the bottom side.Three casters provided with ball bearings, of which only two casters 123and 124 are shown, slide along these steel rails 121 and 122 and carrythe carriage 6. inside the aluminium profile there are two cavities, therear one 125 of which having space for coupling terminals, transformersand similar. The ends of the profile are covered with end closures 126and 127 (see FIG. 1) and there there are attachments for fixing theprofile to ceiling, wall or on a floor stand. In a front cavity 128there runs a cable chain in which power and signal cables lie. The cableexits via a slot 129 in the profile 120. Said slot 129 is covered by atoothed belt 130 (see FIG. 25). The toothed belt is fixedly attached ateither end of the profile body 120. Said toothed belt 130 is lifted awayfrom the slot 129 as the carriage moves by means of a mechanism which isin the carriage and which is shown in FIG. 25. A motor 131 in themeasuring carriage 6 draws the measuring carriage back and forth alongthe profile 120 (see also the reference numeral 7 in FIG. 1) via anexchange 132 and a gear 133. The gear 133 engages with the toothed belt130, and the toothed belt is guided over guiding casters 134, 135 and136. A holder 137 projects into the profile 120 in the cavity 128 andguides the cables into the cable chain and draws the end of the cablechain with it.

By means of the present invention, a total measuring system is thusdescribed wherein a special feature is the design of the double curvedmirror, whereby this is given a focusing power which enables the lenssystem to be moved effectively much closer to the object which is to bemeasured. The solid angle with which one can observe the laser spot willincrease, for a given area, inversely proportional to the square of thedistance. It is therefore essential to make the distance that the laserbeams move as short as possible and thereby achieve the best possibleaccuracy of measurement. Furthermore, the necessary dimension is reducedfor the subsequent components which are positioned after the firstfocusing mirror, especially folding mirror, polygonal mirror and lenseswhich are mounted right in front of the sensor unit.

Even though the invention here has be illustrated and described withreference to preferred embodiment examples, it will immediately beunderstood that modifications can be made within the frame of the patentclaims which follow without thereby deviating from the inventive idea.

Technical equivalents of this kind are thus considered to lie within thescope of protection of the patent claims hereinbelow.

What is claimed is:
 1. An apparatus for measuring the distance to one ormore areas of an object in order to determine one or more dimensions ofthe object, comprising means for directing a light beam towards theobject, means for registering the light beams that are reflected fromthe object, means for calculating the time it takes for the beams totravel to and from the object, and means for causing the light beam tosweep across the object perpendicular in relation to a fixed referenceplane during the entire sweeping operation, and with relative movementbetween the apparatus and the object which is to be measured, said lightwhich is reflected by the object returning back to the lightregistration means via said light beam sweeping means, and said lightbeam sweeping means including a stationary mirror which is arced in itslongitudinal direction, and also rotatable mirror means, comprisingafirst mirror, a second, plane mirror, a third plane mirror which formsand angle of 90° with the second mirror, said rotatable mirror meansbeing made of a rotating polygonal mirror unit having a rotor means, inthat the light beam which is emitted from a light generator has a pathof movement via the first mirror, thence via the polygonal mirror unit,thence towards the second mirror, thence to the third mirror, thence tothe stationary mirror, thence towards the object, the light beamreflected from the object having a path of movement back to the lightregistration means via the stationary mirror, said third and secondmirrors, and the polygonal mirror unit, said stationary mirror in thelongitudinal direction having an arced cross section so that the mirrorforms a double concave mirror surface, the radius of curvature of thearc changing along the length of the stationary mirror and beinggreatest at the ends of said mirror.
 2. An apparatus according to claim1, wherein the rotating polygonal mirror unit is arranged at a distanceabove the stationary mirror.
 3. An apparatus according to claim 1,wherein said second mirror (1) and the rotating polygonal mirror unit(2) are located on a first (1) level and said third mirror and saidstationary mirror (2) are located on a second level.
 4. An apparatusaccording to claim 2, wherein said second mirror (1) and the rotatingpolygonal mirror unit (2) are located on a first (1) level and saidthird mirror and said stationary mirror (2) are located on a secondlevel.
 5. An apparatus according to claim 1, wherein the double concavemirror surface spans over an arc that is less than 180°.
 6. An apparatusaccording to claim 1, wherein the pitch of the stationary mirror,comprising a distance between a chord drawn between the ends of themirror and to the mid-point of the mirror is appreciably less than thesmallest radius of curvature of the mirror by about 15-30% of saidradius of curvature.
 7. An apparatus according to claim 1, wherein atone end of the stationary mirror a fifth mirror is positioned whichdeflects received light beams from the light generator along the path ofmovement of the apparatus in the direction of a reference plate havingareas of different reflective properties, and which is securely fixed toone end of the rail system along which a carriage for the apparatus canmove, the time it takes the light beam to travel from said referenceplate to the light registration means being a function of the positionof the apparatus/carriage relative to the rail system.
 8. Art apparatusaccording to claim 1, wherein the rotatable mirror means being arotating polygonal mirror unit with individually adjustable mirrors. 9.An apparatus according to claim 8, wherein each mirror along a firstedge thereof is mounted in a respective receiving groove on said rotormeans, said groove having its axis located transverse to the axis ofrotation of said rotor means, and wherein each said mirror along asecond edge thereof parallel to said first edge is adjustably attachedto said rotor means by means of a spring element in the form of a metalplate, said spring element being adjustable by means of a screw/nutconnection which is attached to said rotor means at one end and at theother end engages said spring element, said spring element having itsbending curvature changed upon adjustment of said screw/nut connectionto cause said mirror to be moved correspondingly in a direction towardsor away from a mid point of said polygonal mirror unit.
 10. An apparatusaccording to claim 9, wherein said metal plate has a first portionextending over said second edge and engaging a front face of said mirrorand a second portion in the form of two punched-out tongues having thefree ends thereof bearing against a rear face portion of said mirror.11. An apparatus for measuring the distance to one or more areas of anobject in order to determine one or more dimensions of the object,comprising means for directing a light beam towards the object, meansfor registering the light beams that are reflected from the object,means for calculating the time the beams take to travel to and from theobject, and means for causing the light beam to sweep over the objectperpendicular in relation to a fixed reference plane during the entiresweeping operation, and with relative movement between the apparatus andthe object which is to be measured, the light that is reflected by theobject returning to the light registration means via said light beamsweeping means, and said light beam sweeping means comprising astationary mirror which is arced over the length thereof, and meanscausing the light beam to move over the stationary mirror, wherein thestationary mirror has an arced cross-section in its longitudinaldirection such that the mirror forms a double concave mirror surface,and wherein the radius of curvature of the arc changes along the lengthof the stationary mirror, and is greatest at the ends of said mirror.12. An apparatus according to claim 11, wherein the double concavemirror surface spans over an arc that is less than 180°.
 13. Anapparatus according to claim 11, wherein the pitch of the stationarymirror, comprising a distance between a chord drawn between the ends ofthe mirror and to the mid-point of the mirror is appreciably less thanthe smallest radius of curvature of the mirror by about 15-30% of saidradius of curvature.
 14. An apparatus according to claim 13, wherein thepitch of the stationary mirror, comprising a distance between a chorddrawn between the ends of the mirror and to the mid-point of the mirroris appreciably less than the smallest radius of curvature of the mirrorby about 15-30% of said radius of curvature.
 15. An apparatus accordingto claim 11, wherein at one end of the stationary mirror a fifth mirroris positioned which deflects received light beams from the lightgenerator along the path of movement of the apparatus in the directionof a reference plate having areas of different reflective properties,and which is securely fixed to one end of the rail system along which acarriage for the apparatus can move, the time it takes the light beam totravel from said reference plate to the light registration means being afunction of the position of the apparatus/carriage relative to the railsystem.
 16. An apparatus for measuring the distance to one or more areasof an object in order to determine one or more dimensions of the object,comprising means for directing a light beam towards the object, meansfor registering the light beams that are reflected from the object,means for calculating the time it takes for the beams to travel to andfrom the object, and means for causing the light beam to sweep acrossthe object perpendicular in relation to a fixed reference plane duringthe entire sweeping operation, and with relative movement between theapparatus and the object which is to be measured, said light which isreflected by the object returning back to the light registration meansvia said light beam sweeping means, and said light beam sweeping meansincluding a stationary mirror which is arced in its longitudinaldirection, and means causing the light beam to move over the stationarymirror, wherein the stationary mirror in the longitudinal direction hasan arced cross section so that the mirror forms a double concave mirrorsurface, the radius of curvature of the arc changing along the length ofthe stationary mirror and being greatest at the ends of said mirror, andwherein said means for moving the light beam is a rotating polygonalmirror unit.
 17. An apparatus according to claim 16, wherein the doubleconcave mirror surface spans over an arc that is less than 180°.
 18. Anapparatus according to claim 16, further comprising a first mirror, asecond plane mirror, a third plane mirror which forms an angle of 90°with the second mirror, the light beam which is emitted from a lightgenerator having a path of movement via the first mirror, thence via thepolygonal mirror unit, thence towards the second mirror, thence to thethird mirror, thence to the stationary mirror, thence towards theobject, the light beam reflected from the object having a path ofmovement back to the light registration means via the stationary mirror,said third and second mirrors, and the polygonal mirror unit.
 19. Anapparatus according to claim 16, wherein the rotating polygonal mirrorunit is arranged at a distance above the stationary mirror.
 20. Anapparatus according to claim 18, wherein said, second mirror and therotating polygonal mirror unit are located on a first (1) level and saidthird mirror and said stationary (2) mirror are located on a secondlevel.
 21. An apparatus according to claim 19, wherein said secondmirror and the rotating polygonal mirror unit are located on a firstlevel and said third mirror and said stationary mirror are located on asecond level.
 22. An apparatus according to claim 16, wherein thedistance between a chord drawn between the ends of the stationary mirrorand to the mid-point of the stationary mirror, which corresponds to thepitch of the stationary mirror, is appreciably less than the smallestradius of curvature of said stationary mirror and on the order ofapproximately 15-30% of said radius of curvature.
 23. An apparatusaccording to claim 16, wherein at one end of the stationary mirror afifth mirror is positioned which deflects received light beams from thelight emitter along the path of movement of the apparatus in thedirection of a reference plate having areas of different reflectiveproperties, and which is securely fixed to one end of the rail systemalong which a carriage for the apparatus can move, the time it takes thelight beam to travel from said reference plate to the light registrationmeans being a function of the position of the apparatus/carriagerelative to the rail system.
 24. An apparatus according to claim 16,wherein the rotating polygonal mirror unit has individually adjustablemirrors.
 25. An apparatus according to claim 24, wherein each mirroralong a first edge thereof is mounted about an axis located transverseto the axis of rotation of said polygonal mirror unit, and wherein eachsaid mirror along a second edge thereof parallel to said first edge isadjustably attached to a rotor means of said polygonal mirror unit bymeans of an adjustable spring element.
 26. An apparatus according toclaim 25, wherein a the spring element is adjustable by means ofscrew/nut connection which is attached to said rotor means at one endand at the other end engages said spring element, said spring elementhaving its bending curvature changed upon adjustment of said screw/nutconnection to cause said mirror to be moved correspondingly in adirection towards or away from a midpoint of said polygonal mirror unit.